The long term goal of this study is to define the structural and compositional factors relating to the caries susceptibility or resistance of the tooth mineral. (The tooth mineral is a non-stoichiometric calcium hydroxyapatite, modified by the presence of minor constituents (e.g., CO32 minus, Mg2 ion, F minus, Cl minus, HPO42 minus). Studies on structural factors include: crystallite size, morphology, lattice parameters, unit-cell volume, internal strain. Studies on compositional factors will focus on the effects of some minor constituents and some 'trace elements' (suspected of being cariogenic or cariostatic) on the formation and stability of the apatites. Structural factors will affect physical properties such as thermal stability and rate of dissolution, properties which are relevant to the resistance or susceptibility of the dental apatite to acid dissolution or caries. Biological materials will include teeth from different mammalian species; human teeth of known ages; human teeth of known trace elements contents obtained from regions of high and low caries incidence; fluoridated and non-fluoridated fish enameloids and fluorosed human teeth. Synthetic apatites will be prepared from aqueous systems in the presence of the 'trace element' or minor constituent investigated. For the characterization of the biological and synthetic materials, X-ray diffraction, infrared absorption spectroscopy, atomic absorption spectrophotometry, thermo-gravimetry (TGA), and in some cases, transmission (TEM) and scanning (SEM) electron microscopy will be employed.